Attempt `dask_drmaa` import globally
Attempt `dask_drmaa` import globally

To avoid repeatedly importing `dask_drmaa` inside functions, simply
import `dask_drmaa` globally and set `dask_drmaa` to `None` if the
import fails for some reason. This way `dask_drmaa` can be tested for
its truthiness. Use this logic in `get_client`, to handle the
`DRMAACluster` and `LocalCluster` cases.

DudLab/nanshe_workflow,nanshe-org/nanshe_workflow